The present invention relates to disc brakes for vehicles, and in particular to a system and method for mounting and retaining brake pads in disc brakes, such as air-operated disc brakes utilized on commercial vehicles.
Pneumatically-operated disc brakes have been undergoing development and deployment on commercial vehicles since at least the 1970's, and are beginning to replace drum-style brakes due to advantages in areas such as cooling, fade resistance and serviceability. German Patent Publication No. DE 40 32 886 A1 discloses an example of such an air disc brake. In this design, a pneumatic diaphragm chamber (pneumatic actuator) is attached to a rear face of the disc brake caliper housing, and applies a brake actuation force through a linear actuator rod to a brake actuator lever within the caliper. The brake's actuator lever in turn transfers and multiplies the force applied by the actuator rod to one or more spindles, which force brake pads against a brake disc or rotor. The terms “brake disc,” “rotor” and “brake rotor” are used interchangeably herein.
As shown in FIG. 1 of DE 40 32 886 A1, the actuator is located inboard of the brake caliper, in large part because commercial vehicle wheel rims are sized to only provide adequate clearance for the drum-type brakes historically employed on such vehicles. Because the resulting space envelope between the wheel and its axle is limited, the actuator must be located into the space adjacent to the wheel. For the same reason, brake pads must be configured to conform to the limited available radial space, and thus have typically been located and retained on one of the brake caliper or brake caliper carrier/mount using transverse suspension pins and/or using leaf spring-type metal strips disposed over the outer radius of the brake pads. Brake pads have also been retained by capturing the brake pads between the caliper mounting frame and the portion of the brake caliper which straddles the brake disc. Brake pad support function may be provided by a brake caliper carrier/mount designed to support the brake pads or by a brake pad carrier which is separate from the caliper mounting structure. For convenience in this description, the terms caliper carrier, caliper mount and brake pad carrier may be interchanged without intending to limit the brake par supporting structure to any specific brake pad and brake caliper carrying structure.
Conventional commercial vehicle air disc brakes have typically required the installation of ancillary brake pad retention mechanisms such as a retaining bar arranged transversely across the top of the brake pads. Such arrangements require additional space above the brake pads, which in turn requires taller brake calipers. With the opposing constraint of little available clearance between the inside of the wheel rim and the outer envelope of the brake caliper, it is difficult to accommodate such brake pad retention mechanisms without thinning the caliper and thereby increasing the stress levels in the caliper material and decreasing the caliper's useful service life (e.g., decreased fatigue life).
A further problem with prior art brake pads is the tendency for the brake pad to rotate and/or vibrate during brake operation. When a brake pad is applied against a friction surface of a rotating brake disc, the brake disc's rotation induces motion and reaction forces between the brake pad and its adjacent carrier abutment surfaces. Specifically, at the leading edge of the brake pad, the brake pad attempts to move upward relative to the carrier in response to the friction forces along the face of the brake pad, while at the trailing edge of the brake pad, the brake pad attempts to move downward. However, because the brake pad is constrained by adjacent mount abutment surfaces, the overall motion of the brake pad is generally a rotation about an axis parallel to the brake disc rotation axis. This motion may be unilateral during the brake application, or may manifest itself as a moderate-to-severe oscillation of the brake pad in its mount, significantly increasing wear of the abutting brake pad and mount surfaces.
In order to prevent undesired rotation and/or vibration of the brake pad within its mounting (for example, rotation about the brake application direction), the brake pad backing plate and the adjacent mounting bracket horns supporting the brake pads in the circumferential direction required a relatively tall radial height to minimize the amount of brake pad rotation before a corner of the backing plate contacted the adjacent mount horn (a motion referred to as “pad kick” or “pad turnout,” caused by “twisting” or “tipping” of the pad in the caliper as the rotating brake disc attempts to raise one end of the brake pad while pushing down on the other end of the brake pad). This relatively tall structure in turn would require the brake caliper, which is installed over the brake pads and mounting bracket, to have its corresponding opposing inner surfaces radially outboard of the mount horns be relieved enough to accommodate the outer corners of the brake pad and/or mount horns. This thinning again may cause stress level and fatigue life problems. Because the maximum outer radius of the brake caliper is typically constrained by very tight clearance to the inside of the adjacent wheel rim, the brake caliper arms straddling the brake disc between the application side and the reaction side of the caliper may have to be made thinner than desired in order to accommodate both the tall carrier mount horn and the close-fitting wheel rim. This can lead to very high tensile and bending stresses in the thin regions and thus reduction in fatigue life and service life.
One approach to addressing such problems is disclosed in U.S. patent application Ser. No. 13/673,404, the teachings of which are incorporated by reference herein. This patent application discloses brake pads with features to receive spring elements at the pads' lateral and/or lower (i.e., radially inner) sides. The spring elements are configured to cooperate with corresponding features on the brake pad holder to retain the brake pad within the brake during operation, with or without other brake pad retaining features and/or additional brake pad retention devices.
The present invention improves on the spring element approach to brake pad mounting and retention with minimum pad kick motion and pad vibration. In one embodiment of the present invention a spring element configured to engage a lateral and/or lower surface of a brake pad includes a feature such as a protruding “bump” or coiled portion that is shaped to engage within a corresponding spring retaining element such as a bushing in the brake pad carrier. The spring retaining element is configured to resist extraction of the spring element from the retaining element in a radially-outward direction during normal brake operation, and hence resist extraction of the brake pad which is engaged by the spring element in the same direction. The use of a spring retaining element, preferably a replaceable spring retaining element, further improves durability and longevity of the brake pad carrier by isolating the carrier itself from wear caused by brake pad and spring clip motions, as well as removing a stress-concentrating sharp corner where the carrier's horn and bottom surfaces meet.
The spring retaining element may be in the form of a generally cylindrical bushing that is located in a corresponding axial slot in the brake pad carrier, where “axial” refers to the axial direction defined by the rotation axis of the brake's brake disc when the brake is in an installed position on a vehicle axle. The bushing may have an axially-aligned slot which is wide enough to permit a thin section of the spring element to be located between the brake pad and the interior region of the bushing, but narrow enough to preclude removal of a thicker section of the spring element in the radially outward direction (for example the spring element may have a coiled end sized to permit its insertion in the axial direction into the interior region of the bushing). The spring retaining element may be configured to receive the thicker portion of the spring element in the axial and/or radial directions, such as with a brake pad and spring element inserted into the brake pad carrier in either the radially-inward direction or in the axial direction (i.e., toward or away from the brake disc).
The invention is not limited to cylindrical spring element retention arrangements, but may be any shape that provides for positive retention in the radially outward direction of the spring element and brake pad in the brake pad carrier, for example a spring retaining element or corresponding brake pad carrier receiving feature may have a rectangular cross-section.
Similarly, the invention is not limited to arrangements in which the spring element must be inserted into the spring retaining element in an axial manner. For example the portion of the spring element that enters the interior region of the spring retaining element may be configured to expand after passing laterally through a bushing's axial slot as the assembled brake pad and spring element is inserted in the radially-inward direction, such that the spring element is “snapped” into place in the spring retaining element as the brake pad reaches its installed position. Further, a combination of installation motions may be used, such as insertion of the spring element into the spring retaining element, followed by inserting the brake pad radially-inward into the brake pad carrier to align the brake pad with the spring element and then moving the brake pad axially toward the brake disk until the brake pad engages the spring element.
The invention also is not limited to arrangements in which the spring retaining element is located at a lower corner of the brake pad carrier, i.e., at the junction of the carrier's side and bottom walls, but may be located on the side and/or bottom walls as long as the spring element and brake pad are positively retained in the radially outward direction during normal brake operations.
The spring element may also have side extensions, such as tabs, which conform to the width of the brake pad carrier and serve to both align and guide the brake pad into position during installation and lock the spring element into place against lateral motion toward or away from the brake disc as the brake pad is advanced or withdrawn from the brake disc.
The present invention may be used with or without other radially-outer brake pad retaining devices being present over the brake pads.
With this present invention, during in-service brake operations the brake pad is retained in the brake in a simple and positive manner by the combination of: (i) the spring element being retained by spring retaining element that is fixed on the brake pad carrier, and (ii) brake pad being retained by the spring element, for example by engagement of a spring element protrusion into one or more features (such as a notch) in a lateral side of the brake pad. The spring element protrusion which engages the side of the brake pad is preferably formed in a wedge-shape which resists “camming out” of the brake pad slot during the brake application. Spring elements may be provided on one or both lateral sides of the brake pad.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.